Prior to the advent of digital computers, the tools available to the graphic artist included pencils or pens (generally producing a substantially uniform line), and brushes or airbrushes (producing a controllably nonuniform line, variable by varying the pressure and/or speed applied to the tool).
Digital image processing apparatus is known which provides simulations of these manually operable graphics tools. For example, British Patents 2059625, 2140257, 2113950 and 2147122 describe aspects of the "Paintbox" system available from Quantel Limited. With this system, an operator selects the characteristics of the graphics tool he wishes to imitate and then manipulates a pressure sensitive stylus over a digitising pad to input a desired line. As the stylus is moved over the tablet, the apparatus senses the stylus position and the pressure applied thereto, reads image data from a corresponding mapped area of an image store (e.g a frame buffer) modifies the data in accordance with the sensed pressure, and writes it back into the store. The system is arranged and intended to simulate a conventional graphics tool such as a paintbrush or airbrush, and the artist exerts control over the parameters of the line "drawn" in the image store in the same way, so that the width and other attributes of the line are controlled as the stylus moves, and the stored image data comprises a direct representation of the line itself, corresponding to a manually painted line.
It is known in computer graphics to represents objects as parametric curves, the curve shape being specified and controlled by data representing the positions of points on the curve and the tangents thereat; as disclosed in, for example, "Interactive Computer Graphics", P Burger and D Gillies, 1989, Addison Wesley, ISBN 0-201-17439-1.
In "Hairy Brushes", Strassman, 1986 Siggraph Conference Proceedings (Vol 20, No 4, Page 225-232), a system for emulating paintbrushes of a particular kind is described in which a brush stroke is first defined by data specifying the linear trajectory of the brush (as point positions and tangents), and the pressure applied to the brush (which in turn specifies the width of the stroke normal to the line running along the trajectory), and then the colour profile laterally across the brush stroke is specified by the user defining a profile of individual bristle colours laterally across the brush stroke. It is suggested that profiles could be defined at the start and end of the stroke, and the colour profile along the stroke be interpolated from the end values.
As that system is intended to simulate particular types of existing brush, it makes a distinction between properties of the stroke (its trajectory and its pressure - dictated width) and those of the brush (its colour profile).
WO84/02993 shows a system for generating images, in which an image path is dictated by Bezier control points. It is possible to vary the width of the entire stroke defined by the path, as a whole, but not to provide a varying width along the stroke; the object of that proposal is to create a stroke of uniform width.
U.S. Pat. No. 4,897,638 shows an image processing system in which it is possible to specify a varying stroke width, but only at the control points used to specify the curvature of the path.